1. Field of the Invention
This invention relates to nuclear reactor systems and, more particularly, to methods and apparatus for controlling fluid pressure drop within a nuclear reactor fuel element, and the like.
2. Summary of the Prior Art
To produce useful power from a nuclear reactor, it is necessary to accumulate a sufficient quantity of fissionable material, or nuclear fuel, in a relatively small volume that is known as the "reactor core". Usually, a reactor core comprises an array of thin-walled metal tubes that enclose pellets of uranium dioxide, or other suitable nuclear fuel material. These fuel-loaded tubes frequently are referred to as "fuel rods". For fuel handling and structural integrity purposes, groups of these fuel rods within the core are assembled together into fuel elements. The fuel element fittings engage the individual rods and keep the rods in proper relative position. These fittings maintain a spacing between the fuel rods that permit pressurized water, or some other suitable working fluid, to flow through the reactor core and absorb heat from the rods.
The absorbed heat, of course, is subsequently converted into useful work.
After a year or more of power generation, a sufficient quantity of the nuclear fuel within the rods is consumed to justify removing some of the "spent" fuel elements from the core and replacing them with fresh fuel elements.
Naturally, it is important that the fuel elements in this "core reload" should incorporate all of the most recent technical advances and improvements, while nevertheless matching the nuclear, thermal and hydraulic properties of the older fuel elements that are being replaced. This problem is further aggravated by the possibility that the "core reload" supplier is not necessarily the same source as the manufacturer who produced the fuel elements that are undergoing replacement. In this circumstance, the difficulties of matching the operational characteristics of fuel elements that are based on entirely different design principles become even more pressing.
There is, moreover, a need to insure the structural integrity of the fuel elements at all times. A number of approaches have been taken to provide a satisfactory answer to the problems posed by the need for adequate strength in the fuel elements. In many instances, the fuel rods in a given fuel element are lodged within transversely disposed fuel element grids. Frequently, these grids comprise an array of mutually perpendicular, interlocking flat plates which form individual cells. It is within these cells that the fuel rods are received. Bosses that protrude from the plates grip the adjacent surfaces of the fuel rods and support the rods against the hydraulic and thermally generated forces to which the fuel rods are subjected.
The strength required of a fuel element grid in these circumstances must be balanced against the need to keep at an absolute minimum all materials in the reactor core that do not contribute directly to the fission process. This latter requirement is based on the rather obvious consideration that the probability for generating undesirable debris in the reactor core is in some way related to the mass of material that is within the core.
A further and somewhat more subtle reason for reducing structural core material is the parasitic effect that these materials impose on the reactor's neutron population. In this respect, neutrons, emitted from a fissioning uranium nucleus are absorbed in other uranium nuclei to cause these nuclei, in turn, to fission and thereby release energy and further neutrons. It is these successive second generations of neutrons that continue to propagate the fission process. Clearly, neutron conservation is important to the economical operation of the reactor because wasted neutrons reflect wasted fissionable material, or fuel. Thus, neutrons absorbed in non-fissionable reactor core materials represent a loss and an operating inefficiency.
Consequently, the fuel element supplier, and especially a supplier who is providing a reactor core reload, is faced with the task of resolving a number of difficult and essentially conflicting requirements. In this circumstance, the hydrodynamic characteristics of the used fuel elements must match those of the replacement units. This match--and particularly if the pressure losses imposed on the reactor coolant through the older fuel elements are greater than the pressure losses, or "drop" inherent in the replacement grid design should be achieved without introducing parasitic neutron absorbing materials or possible sources of debris into the reactor core.
With respect to grid strength, it also should be noted that the practice of adding a panel or an additional thickness of metal to the central portions of the surface of the individual cells may actually fail in the design purpose between the weakest point in the cellular structure seems to be at the corners that are formed by the interlocking plates. These panels are subject to an additional disadvantage in that they appear to disturb local coolant flow conditions to a degree that will produce "hot spots", or small areas of exceptionally high temperature due to coolant blockage. The potential existance of these "hot spots", and their possible destructive effect, compels the reactor design to be operated at a power level significantly below that which could be enjoyed in the absence of these local temperature anomalies.
Accordingly, there is a need to provide a fuel element that not only is strong, but also can be adapted to produce a predetermined coolant pressure loss without adding parasitic structure to the reactor core.